The purpose of this study is to define the metabolism of biogenic amines -- i.e. serotonin, histamine, dopamine - in the different morphological types of endocrine cells and in the mast cells in the normal and abnormal human stomach and intestines and in the stomach and intestines of the rat and mouse. Methods employed include light and electron microscopic radioautography, organ culture, amine-fluorescent techniques, and thin-layer chromatography and liquid scintillation counting to separate and identify radioactive metabolites.